kselftest/alsa: pcm-test: Move stream duration and margin to variables

kselftest/alsa: pcm-test: Move stream duration and margin to variables

The duration to stream for and time margin to consider the stream failed
are currently hardcoded values. Move them to variables so they can be
reused and more easily changed.

Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20230620220839.2215057-2-nfraprado@collabora.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>